. . . however, I know the site you are referring to - the Kabalarians believe that they can tell you everything about yourself - your nature, your destiny etc, from your name (for a fee, of course, lol). To this end they have an enormous list of names to attract people to their site. When I did a Google search that was one of three sites which contained the name Sjertsje, the other two referring to people of that name. It's obviously not a common name!
Looking at the form of the name, as I said in the subject, it's probably Dutch. I would guess at the pronunciation being something like "syertzya", but I could be very wrong - no doubt Menke or Indy (our two main Dutch posters) will be able to tell you.